On Board Computer retrofit DIY

Contributed by:

Keith Burrell

The On Board Computer (OBC) option (M659) adds a different tachometer to your car with a small display at the bottom of the tach which displays 6 different functions. It came standard on all Turbos and cars with Tiptronic transmissions. The functions displayed in orange letters, figures and symbols include distance to next fill-up, average MPG, average speed, trip odometer, digital speedometer, and outside temperature. After a couple of months of thinking about trying to add this option and finding all used components, I installed the On Board Computer (OBC) option in my 1996 993 Porsche. Here is a simple write up of how I did the installation.
 
 

 Porsche parts required:

Other parts needed:
 Notes on parts:
    The OBC tachometer, steering column switch, and the steering column switch cover can all be purchased used to save cost.
    The steering column stalk for the OBC is a totally separate piece from the steering column switch and is held on with just two screws and a wire tie so you can buy part 993 613 352 00 01C which is cheaper and just transfer the new OBC stalk to your existing steering column switch regardless of which steering column switch you have.
    The OBC tachometer connector is a 26 pin connector that plugs into the back of the OBC tachometer. I do not believe this part is available from the dealer unless you order the entire wiring harness. The connector is made up of a connector surrounded by a plastic shell held together with a wire tie. The connector part numbers are 944 612 551 00 (connector) and 999 652 542 40 (shell). This same connector is used in a couple of different locations in the car. The only differences are the number and the colors of the wires used on the connector. I used a OBC connector off a 1990-1994 964 w/OBC option. This worked out well because this connector has some unused wires which you will need to use later on. The connector to the central indicator (informer) will also work but you will need to modify the position of some of the wires in the connector and the wire colors will not be correct.
Hooking up the OBC tachometer requires the connection of 19 wires. The standard tachometer only has 7 wire connections.  14 of the wires come from the area directly behind the tachometer and from the steering column switch.  The remaining 5 wires have to be routed from other parts of the car (DME area, temp. sensor, driver’s side kick panel, and the central indicator (informer) ) to the back of the OBC tachometer. Please use a voltmeter to check the continuity of your connections as you go.

 
 

Wire List*
 
From
 To Color**
OBC Pin #    
1 Pin #4 of clock (+12V power)
red
2 Stock tach harness pin #2 (ground) and OBC stalk pin #1 brown
3 Stock tach harness pin #3 (turn signal LH) black/white
4 Stock tach harness pin #4 (high beam) white/black
5 Stock tach harness pin #5 (turn signal RH) black/green
6 Stock tach harness pin #1 (lights) gray/blue
7 Not used -
8 Temp sensor pin #1 blue/green
9 Central indicator (informer) pin #7 beige connector (fuel signal) brown/black
10 Temp sensor pin #2 brown/green
11 Not used -
12 Pin #2 of speedometer (speed signal) white/red
13 Pin #4 of connector behind LH kick panel (door armed signal) green/white
14 Stock tach harness pin #7 (tach signal) orange
15 Stock tach harness pin #2 (ground) brown
16 Not used -
17 Not used -
18 Not used -
19 Not used -
20 Not used -
21 Stock tach harness pin #6 (tach signal) black/violet
22 Pin #6 harness under seat from DME (TI signal) white/green
23 OBC stalk pin #2 white
24 OBC stalk pin #3 green
25 OBC stalk pin #4 blue
26 OBC stalk pin #5 gray

*I only had the factory wiring diagrams for the 1994, 1995 and 1996 model years. I am not sure if there were any change for 1997 and 1998. I do not believe there were any but you may want to check with the wiring diagrams for those years to be sure.

** Wire colors given as color1/color2 means a color1 wire with a color2 stripe. For example a Red/Black wire color means a red wire with a black stripe. The wire colors shown on the list are for reference only you may have different colors in your car depending on the year and what colors you use for your installation.
 
 
 
 

Step 1:

Remove seat and DME cover following DIY - How to change the EPROM chip in the OBC
 
 





 Step 2:

Remove rear connector as shown, pull back tape, and splice into pin #6 (white/green wire) coming from DME (top half of connector). Retape connection with electrical tape when done. Run wire under carpet and up to dash. Use enough wire to reach under dash up to the tachometer. Replace cover and install four M 6x12 or M 6x16 pan head screws for stock appearance (I found the M 6x30 screws called out in the DIY to be too long).
 
 





Step 3:

Remove the driver's side kick panel using a M 4 allen socket to remove the cover from the hood release handle and then pull the panel off. Pull back tape from connector on the left (the larger connector), and splice into the pin #4 (green/white wire) coming from the driver's door (top half of connector). Retape connection with electrical tape when done. Use enough wire to reach under dash up to the tachometer.
 
 





Step 4:

Remove the central indicator (informer) under the dash using a 10mm hex socket. It is held in place by two plastic nuts. Disconnect the beige connector from the unit by pulling back the black locking tab. At this point you need to add a missing pin to the beige connector for the fuel signal at pin #7 (if your unit has this pin then you can skip this step). To take the connector apart remove the electrical tape around the wires, cut away the wire tie, and separate the beige connector shell from the white center connector assembly. Once you have the white center connector assembly free, pull out the green tab of the connector so that a new pin can be added.
 
 

To add a new pin, either use one of the unused pin from a 1990-1994 964 OBC connector (Pins #16, #17, or #20) or from a donor central indicator (informer) or other unit connector. Remove the pin by following the directions above and then use a safety pin or other pointed objected to press down on the connector pin tab while pulling the pin wire. Install the new pin in the your central indicator (informer) and make sure the pin locks in place. Reinstall the green tab in the connector and assembly the connector reversing the steps above.
 
 


 
 





Step 5:

Run two wires from under the dash for connection of the temperature sensor (red and yellow wire show here). I ran the wires through a grommet used for the front hood release handle directly above and to the left of the clutch pedal. I spliced the wires into the wire harness on the left side of the hood going to the left lights. Remove the tape from the harness and splice into the blue/green and brown/green wires. Cover wires with black PVC wire loom tubing if you like. Retape harness with electrical tape when done.
 
 


 
 





Step 6:

Install the temperature sensor behind the left side of the front bumper grill. If you remove the front bumper this step will be easier but I chose to do it without removing the bumper. To remove the bumper follow the DIY - How to remove the front bumper cover.  If not following these steps: remove front turn signal light, bumperette, and fog light following directions in owner's manual. The connector for the temperature sensor is already installed in the car (usually tied around your light wire harness) and is shown below on the left. The OBC temperature sensor installs on bracket above existing temperature sensor for the climate control unit. Simply push the new sensor through the hole and make sure it is fully seated in place. If your bumper is removed this will be easy. If not you need to reach through the fog light opening and install the sensor that way. After the sensor is installed, plug the connector into the sensor. You may also want to check the continuity between the temperature sensor wires installed in the car and the temperature sensor connector before plugging the connector into the sensor.
 
 


 
 





Step 7:

At this step you will remove the steering wheel, airbag, trim cover, and the gauges. First disconnect the battery. Undo the screws holding on the airbag using a Trox T30 socket drive. Disconnect the airbag connector and remove the airbag (store the airbag with the padded side facing up). Remove the nut from the steering wheel using a M 24 hex socket. Turn wheel straight ahead and mark position of steering wheel to steering shaft. Lift off the steering wheel and the spring washer. Undo steering wheel trim cover screws on the front and both sides of the unit. Remove the screws from the steering wheel contact unit but do not remove the unit. Now you can remove all the gauges from the dash by gently pulling and twisting each gauge until it pops out from the dash.
 
 
 
 

Step 8:

Now you can attach the OBC stalk. Like I said above, the OBC is a totally separate piece from the steering column switch and is held in place by two screws and a wire tie so you do not have to remove your entire steering column switch to install the stalk. You can just transfer the OBC stalk from your new column switch to you existing column switch using the screws and a wire tie. You can install the entirely new column switch if you like but that requires removal of the contact unit and your existing steering column switch assembly.
 
 
 
 

Step 9:

Next I took apart the connector at the end of the OBC stalk and extended all five of the stalk wires using red female butt connectors that were partially crimped (extend the wires so they are long enough to route under the dash and up to the tachometer). You could also splice and solder the wires to extend them or choose some other method that works. Once the wires have been extended, run them in the opening directly under the steering column and under the dash.
 
 

Step 10:

Now run all the wires from under the dash up to the tachometer area. There should be a total 10 wires. I labeled the ends of all the wires using masking tape so I would not get confused later. To run the wires from under the dash up to the instrument gauges I added a hole and grommet almost directly under the central indicator (informer). Make sure not to drill through any wires or vent ducts!!! You could also try to run the wires through the existing grommet under dash. I did not think there was enough room to do these but this could vary from car to car.
 
 


 
 


 
 





Step 11:

Splice into pin #4 (red wire) of clock connector harness for the +12V power connection and into pin #2 (white/red wire) of the speedometer harness for the speed connection. Retape connections with electrical tape when done.
 
 





Step 12:

Now we are almost done!!! You now need to splice into the 7 wires from the existing tachometer harness. You can splice into the tachometer harness using plastic splices, solder, or whatever makes you comfortable. Make the wires long enough to allow for routing behind the tachometer. Once all wires were in place, I added male and female butt connectors to all of them and to all of the of the wires on the beige OBC 26 pin connector. 38 connectors in total. You could also choose to eliminate the butt connectors here and just solder the appropriate wires together from the OBC connector and the spliced wires.
 
 





Step 13:

Make sure all of the appropriate wires are connected together using the wire list. Use wire ties and/or electrical tape to bundle and secure the wires together. Route wires to ensure that none of them will get pinched by any gauges, screws, or etc..  Install gauges, contact unit, trim cover, steering wheel (torque to 33 ft lb), and air bag (torque to 7 ft lb) by reversing previous steps. Reattach battery and check operation of OBC Tachometer!!!!!
 
 


 
 
 

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